1. Field of the Invention
The invention is related to a transmission system comprising a transmitter for transmitting an input signal to a receiver via a transmission channel, the transmitter comprising an encoder with an excitation sequence generator for generating a plurality of excitation sequences, selection means for selecting an excitation sequence resulting in a minimum error between a synthetic signal derived from said excitation sequence, and a target signal derived from the input signal, the transmitter being arranged for transmitting a signal representing the selected excitation sequence to the receiver. The receiver comprises a decoder with an excitation sequence generator for deriving the selected excitation sequence from the signal representing the selected excitation sequence, and a synthesis filter for deriving a synthetic signal from the excitation sequence. The present invention is also related to a transmitter, an encoder, a transmission method and an encoding method.
2. Description of the Related Art
A transmission system according to the preamble is known from the paper "Codebook searching for 4.8 kbps CELP speech coder" by W. Grieder et. al. in Communications, Computers and Power in the Modern Environment Conference proceeding, Saskatoon, Canada, May 17-18, 1993, pp. 397-406, IEEE Wescanex 1993.
Such transmission systems can be used for transmission of speech signals via a transmission medium such as a radio channel, a coaxial cable or an optical fiber. Such transmission systems can also be used for recording of speech signals on a recording medium such as a magnetic tape or disc. Possible applications are automatic answering machines or dictating machines.
In modern speech transmission systems, the speech signals to be transmitted are often coded using the analysis by synthesis technique. In this technique, a synthetic signal is generated by means of a synthesis filter which is excited by a plurality of excitation sequences. The synthetic speech signal is determined for a plurality of excitation sequences, and an error signal representing the error between the synthetic signal, and a target signal derived from the input signal is determined. The excitation sequence resulting in the smallest error is selected and transmitted in coded form to the receiver.
In the receiver, the excitation sequence is recovered, and a synthetic signal is generated by applying the excitation sequence to a synthesis filter. This synthetic signal is a replica of the input signal of the transmitter.
In order to obtain a good quality of signal transmission a large number (e.g. 1024) of excitation sequences are involved with the selection. In the case of speech coding an excitation sequence is in general a segment with a duration of 2-5 ms. In the case of a sample frequency of 16 kHz, this means 32-80 samples. The parameters of the synthesis filter are in general derived from analysis parameters which represent characteristic properties of the input signal. In speech coding the analysis parameters used mostly, are so-called prediction parameters. The number of prediction parameters can vary from 10 to 50, and consequently the order of the usual synthesis filter, referred to herein as a "full complexity synthesis filter".
Having to compute the synthetic speech signal for all excitation sequences results in a substantial computational burden.